The present invention relates to a mass spectrometer using an ion trap.
The recent environmental pollution by chemical substances has been a serial social problem. Particularly it is one of the most important and urgent items to find a behavior of dioxin in the environment. The dioxin series are isomers having di-oxyn structures whose hydrogen atoms are substituted by chlorine atoms. Particularly among dioxin series, 2,3,7,8 tetra-chloro di-benzo dioxine (2,3,7,8 TCDD) is a strongest carcinogenic and high toxic substance and its diffusion into the environment is looking serious. The environmental pollutants like the dioxin series are extremely little in much complicated systems. Therefore, high-sensitivity apparatus is required to detect and analyze such a little amount of dioxin series for prevention of dioxin series from diffusing into our environments.
The dioxin series referred to in this invention is a generic name for poly-chloro di-benzo para-di-oxin (PCDDS) and poly-chloro di-benzo furan (PCDFs).
The analysis of such chemical substances requires much complicated and very time-consuming pretreatment. Further this kind of analysis must have extremely high sensitivity and selectivity to identify the target substances among coexisting substances. For this reason, a gas chromatograph mass spectrometer (GC/MS) has been widely used which has a gas chromatographic unit for separating substances before the mass spectrometer.
Recently, ion trap mass spectrometers have attracted considerable attention as mass spectrometers. Typical examples of ion trap mass spectrometers are disclosed by U.S. Pat. No. 2,939,952 and Japanese Patent publication No. 60-32310(U.S. Pat. No. 1,321,036), 8-21365, and Japanese Patent Application Laid-Open No. 10-213566.
One of the greatest problems in analysis of dioxin series is that various kinds of interfering substances (e.g., chlorinated pesticides such as PCB and DDT) are left in samples without being removed by the complicated pretreatment. Such interfering substances cannot be removed even by chromatographic capillary columns, dissolve together with the dioxin series in the same retention time period, and are detected by the mass spectrometer. Such interfering substances in analysis are called chemical noises. The chemical noises appear in all mass range of the mass spectroscopy and make it difficult to identify signals of a trace of dioxin.
FIG. 11 shows a mass spectrum having a magnified molecular region of dioxin. In this spectrum, the white bars are for chemical noises and the solid bars are for dioxin signals for easy recognition. (Actual machines use different indications.) As seen from this spectrum, dioxin signals are overlapped by chemical noises. However, the resolution of the ion trap mass spectrometer.
Chemical noise patterns are dependent upon samples and analyses and chemical noises disturb analysis of dioxin as far as they coexist. A high-resolution double-focusing mass spectrometer employing large-scale magnetic and electric fields are used to pick up dioxin signals from chemical noises according to a trifle mass difference between the dioxin and the chemical noises such as PCB and DDT. However, the high-resolution double-focusing mass spectrometer is very expensive, complicated to operate and requires a long and special experience.
Besides analysis by the high-resolution double-focusing mass spectrometer, various analytical methods have been tried to make the analysis quicker and easier. Typical ones of such methods are analysis by a small mass spectrometer and a MS/MS method by an ion trap mass spectrometer.
Although a sample, for example dioxin, is a chemically-stable compound, it gives a unique cleavage when a Collision Induced Dissociation (CID) is performed on dioxin series by the MS/MS method. From a precursor ion having a mass of M, is produced a daughter ion having a mass of M-63 (by reducing COCl from a molecular ion M+) by cleavage. This is a unique cleavage pattern of the dioxin series (PCDD and PCDF). To check the existence of dioxin, it is required to identify and isolate the precursor ion M, produce the daughter ion M-63 by the CID, and detect it. This MS/MS method can distinguish dioxin signals from chemical noises and increase selectivity of the signals even when the resolution of the ion trap mass spectroscopy is not enough.
However, in spite of this high distinguishing performance, the MS/MS method has demerits shown below.
The MS/MS method comprises a precursor ion isolating step, a CID (Collision Induced Dissociation) step, and a mass spectroscopy step and each step has a problem. In the precursor ion isolating step, the operator must judge whether the target precursor ions are not lost at all or whether the precursor ions are completely isolated (or whether only single-mass ions are left in the ion trapping space or whether ions of the other mass coexist in the ion trapping space). However, the conventional MS/MS method provides no information to help the operator to judge the existence of a loss in isolation of precursor ions and the degree of isolation. Therefore, even when the isolation status changes according to the operating conditions of the machine, the operator cannot know the change during and after the measurement. It is very hard to keep the identical operating conditions of the machine.
The CID causes the excited precursor ions to collide with helium gas atoms and to cleave themselves. Therefore, the CID is a kind of chemical reaction. To exactly know how efficiently this chemical reaction advances is always required for exact quantitative analysis. However, the conventional MS/MS method singly measures standard substances in advance and never provides any information of CID efficiency.
There is another method of using internal standard substances to partially solve the above problems (pertaining to isolation of precursor ions and CID efficiency). Compounds containing stable isotopes are used as internal standard substances because the structures of the standard compounds must not be so different from that of the sample. For example, the internal standard substance for analysis of TCDD is a compound obtained by substituting all carbons of the dioxin structures by 13C. In this case, the mass difference between the sample TCDD and the internal standard substance 13-TCDD is 12. First the sample TCDD is MS/MS-analyzed and then the internal standard substance 13C-TCDD is MS/MS-analyzed. Dioxin isomers having four or more chlorine atoms are very toxic. In other words, there are five dioxin isomers of different masses which are very toxic. In actual dioxin analysis, these five dioxin isomers and five internal standard substances (a total of ten substances) must be MS/MS-analyzed. One MS/MS analysis (measurement) takes about 0.2 second. This means that ten serial MS/MS measurements (a cycle) require 2 seconds. Further, for quantitative analysis of a single ingredient, at least ten chromatographic peaks must be sampled. Sampling of ten points of a single ingredient requires 20 seconds (assuming that 1 cycle takes 2 seconds). The GC chromatography is not good for exact quantitative analysis because a dioxin peak dissolves about 5 seconds. Contrarily, the cycle of measurement must be 0.5 second or less (a cycle). Further, for detailed analysis, measurement of internal standard substances is required. However, this singly prolongs the cycle of measurement and is far from exact quantitative analysis.
FIG. 8 shows steps of a conventional MS/MS method on TCDDs and internal standard substances. Step (1) (Ionization step) ionizes TCDDs and internal standard substances together.
Step (2) isolates a selected ion (m/z332, etc.) of the internal standard substance as a precursor ion. Step (3) cleaves the selected ion m/z332 by CID to produce the daughter ion m/z268 and measures the current of the daughter ion from the mass spectrum. Next, the sample measurement steps follow. Step (4) ionizes the sample TCDD and the internal standard substance, selects and isolates the precursor ion (m/z320, etc.) from TCDD ions (Step (5)). Step (6) cleaves the precursor ion (m/z320) of the isolated TCDD by CID into a daughter ion (m/z257). These steps (1) to (6) are repeated for example, five more times to measure all dioxin isomers having 4 to 8 chlorine atoms.
As stated above, the quantitative analysis using the conventional MS/MS method has demerits that the number of samples to be measured is limited, that the time for measurement is prolonged, and that the operator cannot judge whether the MS/MS method is done correctly.
An object of the present invention is to provide high sensitivity and high reliability mass spectroscopic method for dioxin series and other sample, improving the conventional MS/MS methods.
The present invention is related to amass spectroscopic method using a ion trap mass spectrometer. This method leaves ions of two mass ranges or more in the ion trapping space, excludes the other ions, performs CID on the trapped ions, and detects the currents of the resulting daughter ions.